1. Field of the Invention:
This invention relates to a slit exposure type color image forming apparatus having a transfer means capable of improving the transfer characteristics of a photosensitive material, so as to stabilize the formation of an image in such an apparatus.
2. Description of the Prior Art:
The typical conventional methods of and apparatuses for obtaining copies of documents and drawings include the following. First, an apparatus utilizing the electrophotography is made to be of extremely small dimensions, and has a capability of obtaining monochromatic copies with a high efficiency. However, in order to reproduce a color original, it is necessary to expose it by using blue, green and red color separation filters, develop a latent image, which is formed on a photosensitive member, by using color toners, such as yellow, magenta and cyan, and thereby transfer different color portions of the image on the original to the same recording paper repeatedly by the same steps. This causes the dimensions of the apparatus to increase greatly, and makes it necessary that the apparatus has a high operation-repeating accuracy. Moreover, in this apparatus, the staggering of colors occurs, and a clear image of a high quality can rarely be reproduced.
A photosensitive material consisting of silver halide is superior to other photosensitive materials with respect to the photographic properties including the gradation and color reproducibility. The copiers using such a photosensitive materials have been made public and sold by the Ciba Gigy, Inc. and Kis France, Inc.
A copier utilizing a photosensitive material consisting of silver halide employs a total exposure system in which the exposure is done collectively at once. Accordingly, the space occupied by an optical system becomes large, and the dimensions of the copier as a whole increases greatly, so that the space in which the copier is to be installed also increases. A large light source capable of applying light uniformly to an object is also required, and it is very difficult to obtain a means for maintaining the whole surface of a photosensitive material in a horizontal state.
In the copier disclosed in Japanese Patent Laid-Open No. 204042/1984, the whole of the surface to be printed of a fixed photosensitive material is scanned with an image-exposing beam, and an exposure device having complicated construction and large dimensions is employed as in a copier in which the whole surface of a photosensitive material is exposed simultaneously. In the copier disclosed in this publication, keeping the surface of the photosensitive material horizontal is also difficult.
With a view to solving these problems, miniaturizing a copier as a whole by reducing the sizes of an optical exposure system in a scanning type exposure device used therein may be thought of.
In the scanning exposure, in other words, slit exposure, the exposure of an image is done through a slit of a narrow width. Therefore, when the moving speed of a photosensitive material in an exposure position varies, variations in exposure, i.e. variations in the image density occur, so that a belt-like different-density portion, which extends in the lengthwise direction of the slit, appears in a final image.
These inconveniences occur markedly, especially, in a copier using a photosensitive material which has excellent image reproducibility, such as a photosensitive material of silver halide, and which is capable of reproducing even the images of finely different intermediate gradations.
In all of the above-described conventional color copiers using a photosensitive material of silver halide, a photosensitive material is placed fixedly to carry out the exposure of an image. Therefore, a large space is required for installing an optical exposure system therein, and the dimensions of the copiers become large.
Moreover, it is difficult to set a photosensitive material of a large area, such as a photosensitive material of B-4 size and A-3 size on an image-forming surface with a high accuracy, and design a means for transferring a photosensitive material to an exposure stage, an exposure stage itself and a means for transferring the photosensitive material from the exposure stage to another position.